To make simple syrup, bring 2 cups of sugar and 1 cup of water to a boil. Once the sugar has dissolved, remove from heat. Let cool to room temperature. Combine vodka, limoncello, simple syrup and half-and-half with ice in a cocktail shaker, and shake well. Strain into a martini glass. Top with a dollop of whipped cream. Sprinkle lemon zest on the cream.